Quadrupole rod sets are well known and comprise four rod electrodes. The quadrupole rod set may be operated in an ion guide only mode of operation by applying RF only voltages to the electrodes. In this mode of operation ions are not mass filtered. Alternatively, the quadrupole rod set may be operated as a mass filter or mass analyser by applying a combination of DC and RF voltages to the electrodes and then scanning the voltage(s) applied to the rod electrodes.
Quadrupole rod set ion traps are also known. A short set of quadrupole rods known as “stubbies” may be provided upstream and downstream of the quadrupole rod set in order to provide axial confinement within the ion trap. It is also known in an alternative arrangement to provide an annular electrode upstream and downstream of the quadrupole rod set in order to provide axial confinement within the ion trap. Ions may be resonantly excited from the ion trap by applying a combination of voltages to the electrodes.
It is also known operate a quadrupole rod set as a mass filter or mass analyser by applying just RF voltages to the rod electrodes. In this arrangement a grid electrode is provided downstream of the quadrupole rod set and a DC voltage is applied to the grid electrode. The grid electrode acts as an energy filter. Only ions having sufficient axial kinetic energy are able to overcome the DC potential barrier and be transmitted through the energy filter. Other ions with insufficient axial kinetic energy are reflected by the DC potential barrier. Such ions Invariably impact upon the rod electrodes and are lost to the system.
The known quadrupole rod set mass filter or mass analyser is operated so that ions having desired mass to charge ratios are radially excited and undertake large radial excursions without being lost to the rods. In the exit region of the quadrupole rod set fringing fields are present which cause coupling of the radial and axial energies of ions present in this region. Accordingly, ions having relative large radial energies acquire relatively large axial kinetic energies. Ions having desired mass to charge ratios thus emerge from the quadrupole rod set with relatively large axial kinetic energies and are able to overcome the energy filter and be onwardly transmitted whilst other ions are reflected by the energy filter and are lost to the system.
RF only quadrupole rod set mass filters or mass analysers have particular application in lower cost mass spectrometers. In particular, the mass filter or mass analyser is less expensive than a conventional quadrupole mass filter or mass analyser since there is no requirement to provide a DC voltage supply to the rod sets. Furthermore, the rod electrodes can be relatively short. RF only quadrupole rod set mass filters or mass analysers are therefore particularly useful in miniature mass spectrometers and mass spectrometers which are desired to have a relatively small footprint.
Linear quadrupole ion traps with axial ejection (“LQITWAE”) are also known and are similar to RF only quadrupole rod set mass analysers. An additional entrance electrode is provided upstream of the quadrupole rod set to confine ions axially within the ion trap.
Conventional RF only quadrupole rod set mass analysers suffer from the problem that they have relatively poor transmission and resolution performance.
Linear quadrupole ion traps with axial ejection have relatively better transmission and resolution performance than conventional RF only quadrupole rod set mass analysers. However, it would be desirable to further improve the performance of linear quadrupole ion traps with axial ejection.
It is therefore desired to improve the performance of RF only quadrupole rod set mass analysers and linear quadrupole ion traps with axial ejection.